The download route sets Content-Length from a stat and then streams the
object; when the stat size exceeds the bytes the stream yields (#590
"cause 2"), the client hangs on keep-alive framing waiting for a tail
that never arrives. Both send paths now run through a backpressure-safe
byte-counting Transform that resets the socket on a shortfall, so the
download fails at once instead of hanging. Adds a real-socket regression
test at the generic download route, the coverage gap #590 named.
Refs #590
Co-authored-by: harshjainnn <170849281+harshjainnn@users.noreply.github.com>
